The invention relates to an electrical measuring transformer or a control assembly for a multi-ratio transformer in a power-metering system or in a protective relay arrangement. More particularly, the present invention concerns single- or multiphase transformers, capable of converting an actual electrical magnitude into a compatible digital value by means of a measuring, counting, control or monitoring module.
The invention will find applications in the field of electrical construction of such transformers and, more especially, in the manufacture of current transformers.
In this context, it is known that such current transformers are constituted by a primary circuit and a secondary circuit which give to the secondary circuit a proportionally reduced current which is galvanically insulated from the current flowing through the primary circuit.
Such transformers are utilized to feed measuring, counting, control or monitoring modules. These modules are in general designed to operate with a weak current and therefore require the utilization of measuring transformers when the magnitude of the currents to be controlled is greater than the rated value of said modules which, as a rule, is of the order of five Amperes.
Given the specific applications of certain control and counting modules, the transformers are so constructed that the stepped-down current of the secondary circuit is exactly proportional to the primary current, that is to say, to be the total image thereof. This is particularly important when the counting module serves for the invoicing of the power consumed by a user connected to the national distribution grid.
In such a case, there appear different sources of erroneous invoicing which may be prejudicial either to the consumer or to the distributor.
In fact, in spite of the precision applied to the counting modules and the strict controls to which these are subjected, if the image of the consumed current is not reliable, the counting will be falsified. This may derive from the construction of the measuring transformer as such, but, to an equal extent, from the inadequate adaptation of the transformer to the counting module.
In particular, certain apparatuses function badly or less well below a certain threshold of the secondary current, which is the reason while the latter must then be comprised between a lower limit and an upper limit, in other words, within an operating range characterised by its rated value.
Similarly, depending on the type of module to be fed and, to be more specific, depending on the power consumed by the module thus fed, it is necessary to construct the measuring transformer differently in order to effect a correction of Amperes/revolutions such that the error curve of the transformation ratio, specific to the transformer, be comprised between two values defined by the standards or by the distributor or by the regulating body concerned.
In practice, when the transformer is constructed for use in association with an electro-mechanical module, it is accepted that the power absorbed be of the order of 15 VA. In this case, the error curve of the transformation ratio is comprised within fixed limits. By contrast, if this same transformer were to be used with an electronic counter or other module, the absorbed power will be much lower, of the order of 3 VA, and the error curve would fall outside the permitted limits, which would falsify the measurement.
Other causes of erroneous measurement may also be attributable to the person installing the module, by a faulty wiring or an improper choice in the calibration of the measuring transformer.
In fact, in the case of the control of multiphase, more especially three-phase networks, the measurement of intensity must be carried out on each phase form example by means of a multiphase set. To facilitate terminology, we shall be referring to a "multiphase transformer" in what follows. However, in the case of current measurement, this multiphase transformer shall be composed of "n" single-phase transformers.
In the case of the triphase method, there are employed as a rule three single-phase transformers which are adequately coupled so as to obtain a good measurement. In particular, care must be taken to respect the direction of winding, in order to avoid accidental dephasing, and to ensure an identical selection of the calibration of the three transformers.
With regard to this latter point, in the case of electrical power distribution consumption by the users will differ from one user to another, and it is possible to visualise user networks consuming 50 Amperes while others consume 2000 Amperes. The function of the intensity transformer is to adapt the power consumed to the rated value of the counter, which makes it possible to provide a single type of counter.
However, it is not possible to provide a single type of measuring transformer because, as mentioned above, when working with a transformer rated for 1000 Amperes, it will yield erroneous readings if the consumption is only 50 Amperes, due to the fact that its operating range is characterized by its rated value.
A study of the adequacy of these respective operating ranges has shown that there is employed in general practice a range of six current transformers, covering practically all requirements, with transformation ratios of 10, 20, 40, 100, 200 and 400 for 5 Amperes on the secondary winding.
This being the case, it is necessary to hold in stock or to utilize one of these six ratios. Moreover, it is frequently found that,in a three-phase set, one of the transformers used is not identical to the two others.
To the above enumerated disadvantages must be added an operational drawback, taking into account the temporal evolution of the consumption on the network.
In fact, it is a frequent occurrence in power-metering practice that with time the user increases his consumption and demands the modification of the rating of his counter. In this case, it is necessary to intervene at the level of the distribution board and to replace all the measuring transformers.
At present, no device exists which would remedy these different disadvantages, and the good functioning of the installations essentially depends on human control.